The manufacture of rigid hardboard sheets generally involves compressing fiberboard mats with heat and pressure and the inclusion of one or more adhesives or binders. This process involves no particular problems when flat uniformly thick planar hardboard sheets are produced. However, the production of three dimensional non-planar hardboard panels, especially door facings, from fiberboard mats by matched die set molding causes stretch marks and fractures in deeply molded areas or regions adjacent thereto. In a molding press, the tension and compression applied to the mat by the die set pull and push the fibers of an internally self-supporting fiberboard mat apart, sometimes to the breaking point. This is a particularly significant problem with fiberboard mats which have little or no resinous binders which would flow in response to said forces to fill the space around the contours and angles of the die set to which the relatively inelastic fibers could not flow.
The described problem is particularly troublesome when the fiberboard mat used in the molding process is produced by the water felted process. The resulting mat, which contains a minimum amount of a binder or resin, is sufficiently stiff or semi-rigid, due to its integral strength, as to be substantially self-supporting thereby permitting it to be handled manually without a separate support. However, the inherent stiffness or semi-rigidity of the mat prevents the production of high quality non-planar hardboard from such a mat by die set molding. A need accordingly exists for apparatus and methods which can be used to condition such mats, as well as mats made by other than the water-felted method, so that they can be used in die set molding to yield high quality hardboard three dimensional panels.